Changing out large old mill motor for vfd install?

[Karl_T]

That old motor is extremely well made, should last forever. Only thing you might double check is the bearings. With no belt attached, spin it by hand. If it runs smooth and quiet they are fine. This is about the only thing that can break on them.

I also have a a two speed motor on a VFD. This one runs either 1800 or 3600, yours may be different. Especially since you mention it is huge. Lower RPM motors have more mass.

Anyway, I leave mine wired on the low speed for more torque. Then run the VFD from 30 Hz to 130 Hz to give motor speeds from 900 to 4000 RPM

 

[Net30]

That old motor is extremely well made, should last forever. Only thing you might double check is the bearings. With no belt attached, spin it by hand. If it runs smooth and quiet they are fine. This is about the only thing that can break on them.

I also have a a two speed motor on a VFD. This one runs either 1800 or 3600, yours may be different. Especially since you mention it is huge. Lower RPM motors have more mass.

Anyway, I leave mine wired on the low speed for more torque. Then run the VFD from 30 Hz to 130 Hz to give motor speeds from 900 to 4000 RPM

I was surprised to see more horsepower at higher speed, seems counterintuitive

 

[Ulma Doctor]

I was surprised to see more horsepower at higher speed, seems counterintuitive

You may be confusing Torque with HP .
Horsepower is the measure of the work that can be done in a measured timeframe .
Torque is the twisting force around an axis, we measure it in lb ft.

Think top fuel dragster …
Their 8+ liter engines spin over 10,000 rpms and develop 1000’s of HP, but small amounts of torque relative to the HP rating (hundreds of foot lbs)
Then think Big Rig - 8+ liter engines spinning at 1900 rpms and develop 500HP , but can develop nearly 1600 foot lbs. of torque .

The analogy is very similar to your electric motor , the difference is rotor (crankshaft) potential , the slower spinning motor will have a greater torque available, the faster spinning motor will have more horsepower available

 

[Beckerkumm]

Most likely the two speed motor, if 1800-3600 or 900-1800 does not have two separate windings . Look up Dahlander connected motors. The motor will generally have four poles and the Dahlander connection drops two poles to increase rpm. The motor will be rated as 1 hp @1800 rpm and 2 hp at 3600 rpm with constant torque at both speeds. If so, running at low speed doesn't provide any more torque and risks the old motor when running at high hz. I run old motors on a vfd but try to stay under 90 hz ( actually under 80 hz ). The single speed 3 hp will deliver more torque but even an inverter rated motor can have issues at below 25-30 hz. A vector duty motor with a constant torque ratio of 1000-1 is a better candidate than a typical 10-1 inverter motor for a lathe with no back gear as it holds torque from 0hz and can run all day at 120hz or more although any speed over 90 hz comes at a cost of Hp and Torque.

I'd probably run the existing motor at high and see how it works. If you want more torque at low speed, convert to the 3 hp.

Dave